Lignin-zinc oxide nanohybrid emulsion for UV protection

ABSTRACT

A lignin-zinc oxide nanohybrid may be formed by sonication of isolated lignin derived from  Phoenix dactylifera  biomass in an aqueous solution of a soluble zinc salt. The lignin-zinc oxide nanohybrid emulsion or nanoemulsion may then be formed by mixing the lignin-zinc oxide nanohybrid with oil and a stabilizing surfactant and sonicating. The lignin-zinc oxide nanohybrid emulsion effectively bocks UV radiation across the UV spectrum and might therefore be used for UV protection as a sunscreen.

BACKGROUND 1. Field

The disclosure of the present patent application relates to UVprotection, and more particularly, to a lignin-zinc oxide nanohybridemulsion for UV protection.

2. Description of the Related Art

Exposure to ultra violet (UV) radiation (i.e., radiation withwavelengths between 100-400 nm) leads to health risks and injury, suchas cataracts, photoconjunctivitis, early skin aging, inflammation in thedermis, other skin problems, and carcinogenesis. UV radiation may besubdivided by wavelength into UVC (200-290 nm), UVB (290-320 nm) and UVA(320-400 nm). Sunscreens available in the market can protect fromhazardous UV radiation. Generally, sunscreens are divided into twocategories that are based on their mechanism of protection, viz.,physical or chemical. However, long-term usage of commercially availablesynthetic chemical-based sunscreen can cause adverse health andenvironmental effects. Naturally available plant materials includingCarica papaya extract, Rosa kordesii extract, Helichrysum arenariumextract and green coffee oil have the potential to protect against UVradiation with reduced adverse health and environmental effects.However, such natural sunscreens do not shield the full UV lightspectrum. In addition, natural phytochemicals are expensive, and largescale manufacturing of such materials has more limitations thansynthetic chemical-based sunscreens.

sunscreens do not shield the full UV light spectrum, naturalphytochemicals are expensive and large scale manufacturing of suchmaterials has more limitations than synthetic chemical based sunscreens.

Lignin is a major biopolymer found in plant cell walls. Lignin is anaromatic three-dimensional, amorphous polymer substance. Generally,lignin structure includes coniferyl alcohol, p-coumaryl alcohol andsinapyl alcohol moieties. Lignin structure differs based on the sourceplant, source plant parts, isolation method and plant environment.Lignin is a sustainable, biodegradable, environmentally benign andlow-cost material. Annually, around 50 million tons of lignin isgenerated by paper industries. But, 98% of such lignin waste iscombusted as fuel, contributing to pollution through greenhouse gasemission. Alternative uses for lignin are therefore desired.

Thus, a lignin nanohybrid nanoemulsion solving the aforementionedproblems is desired.

SUMMARY

Lignin nanohybrids are synthesized by mixing lignin with water anddispersing by ultrasonication. The lignin may be derived from Phoenixdactylifera (date palm) biomass. In particular, the lignin may beextracted from Phoenix dactylifera biomass by a hydrothermal extractionfollowed by acid precipitation. A zinc salt is then added to the lignindispersion under stirring, followed by sonication, centrifugation anddrying to form a pellet comprising the lignin nanohybrids comprisinglignin and ZnO, alternately referred to as a lignin/ZnO nanohybrids, aswill be demonstrated in the following Examples.

The lignin nanohybrids and ligni nanohybrid nanoemulsion effectivelyblocks transmission of UV irradiation, particularly over oil or ligninalone. Thus, a method of protecting against UV irradiation may includeapplying the lignin nanohybrids or the lignin nanohybrid nanoemulsion toa surfact to be protected.

These and other features of the present subject matter will becomereadily apparent upon further review of the following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is the UV-Vis spectrum of lignin/ZnO nanohybrids synthesized asdescribed herein.

FIG. 1B is the UV-Vis spectrum of a lignin/ZnO nanohybrid emulsionsynthesized as described herein.

FIG. 2 is a composite diffractogram comparing the X-ray Diffraction(XRD) pattern of lignin with the XRD pattern of lignin/ZnO nanohybrids.

FIGS. 3B and 3A are the Energy Dispersive X-Ray Spectroscopy (EDX)spectrum of lignin/ZnO nanohybrids and a SEM micrograph of thecorresponding area mapped by the spectrum.

FIGS. 4A, 4B, 4C, and 4D are EDX elemental mapping micrographs of C, Oand Zn in lignin/ZnO nanohybrids synthesized as described herein.

FIGS. 5A and 5B are SEM micrographs of lignin/ZnO nanohybrids.

FIG. 6 is a plot of particle size distribution of lignin/ZnO nanohybridsas shown by Dynamic Light Scattering (DLS).

FIG. 7 is a comparison of UV shielding of lignin/ZnO nanohybrids andlignin/ZnO nanohybrid emulsion synthesized as described herein as shownby a plot of percent transmittance as a function of wavelength.

FIG. 8 is a comparison of UV shielding of coconut oil alone and ligninalone as shown by a plot of percent transmittance as a function ofwavelength.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Lignin nanohybrids are synthesized by mixing lignin with water anddispersing by ultrasonication. The lignin may be derived from biomass ofPhoenix dactylifera (date palm), and the biomass may be derivedprimarily from leaves (i.e., leaflet, rachis, stalk, petiole andspines). The lignin may be extracted from Phoenix dactylifera biomass bya hydrothermal extraction followed by acid precipitation. A zinc salt isthen added to the lignin dispersion under stirring, followed bysonication, centrifugation and drying to form a pellet comprising thelignin nanohybrids comprising lignin and ZnO, alternately referred to asa lignin/ZnO nanohybrids, as will be demonstrated in the followingExamples. The zinc salt may be zinc acetate.

A lignin-zinc oxide nanohybrid emulsion is synthesized by mixing thelignin nanohybrids with water and dispersing the lignin nanohybrids byultrasonication to form a dispersed lignin nanohybrid solution. Thedispersed lignin nanohybrid solution is mixed with oil to form a firstmixture. A surfactant is added to the first mixture to form a secondmixture, which is then sonicated to form a lignin nanohybridnanoemulsion.

Both the lignin-zinc oxide nanohybrids and the lignin-zinc oxidenanohybrid emulsion effectively block transmission of UV irradiation,particularly when compared to oil or lignin alone. Thus, a method ofprotecting against UV irradiation may include applying the lignin-zincoxide nanohybrids or the lignin-zinc oxide nanohybrid emulsion to asurface to be protected. The surface to be protected may be anorganism's skin.

Generally, plant biomass includes hemicellulose, lignin, and cellulose,among other components. However, the composition of plant biomass variesfrom one plant to another plant. The date tree biomass has a highconcentration of cellulose (39-47%) and lignin (32-35%) and a lowerconcentration of hemicellulose (15-25%). Interestingly, a date palmleaflet includes 41-43% cellulose and 32-36% lignin. Moreover, a datepalm leaf stalk includes 44-47% cellulose and 33-38% lignin. Comparedwith other parts of date palm, the leaflet and leaf stalk-based wastegeneration is very high and has high cellulose and lignin content. Thus,date palm biomass is highly suitable for synthesis of cellulose, lignin,and carbon-based nanostructures.

It should be understood that the amounts of materials used to illustratethe methods described herein are exemplary, and appropriate scaling ofthe amounts is encompassed by the present subject matter, so long as therelative ratios of materials are maintained. As used herein, the term“about,” when used to modify a numerical value, means within ten percentof that numerical value. As used herein, the prefix “nano”, in thecontext of particles or emulsions, refers to a characteristic size ofthe particles or emulsions being within the range of 1-999 nm.

The following examples illustrate the present disclosure.

Example 1 Synthesis of Lignin-Zinc Oxide Nanohybrids and Lignin-ZincOxide Nanohybrid Emulsion

A sample of Phoenix dactylifera biomass (composed primarily of leaves,i.e., leaflet, rachis, stalk, petiole, and spines) was collected fromthe King Saud University Campus, Riyadh (Saudi Arabia). The collectedbiomass was dried in a dark, room temperature environment, cut intosmall pieces, and pulverized in a blender to produce a biomass powder.About 25 g of biomass powder was immersed in 4% of 1 L sodium hydroxidealkali aqueous solution (40 g sodium hydroxide in 1 L water), and thebiomass in solution was autoclaved at 120° C. for 2 hours under 15 lb ofpressure, resulting in a black liquor with solids. The black liquor wasseparated by filtration through Whatman filter paper. The black liquorwas pH adjusted to a pH of 2.0 using hydrochloric acid and kept at roomtemperature until a black precipitate formed in the pH-adjusted blackliquor. The black precipitate was isolated by centrifugation. Theisolated black precipitate was dried at room temperature to produceisolated lignin.

Then, 100 mg of the isolated lignin was mixed with 50 mL of distilledwater and dispersed using ultrasonication. About 50 mL of 0.001 M zincacetate solution (in water) was added to the dispersed lignin solutionunder stirring to form a zinc/lignin mixture. The zinc/lignin mixturewas sonicated for 45 minutes using probe sonication at 750 W.Subsequently, the mixture was centrifuged at 20,000 rpm for 15 minutes.The resulting pellet was isolated by removing supernatant and dried atroom temperature. The resulting dried pellet comprises the exemplarylignin-zinc oxide nanohybrids.

Either 50 mg of isolated lignin (as control) or the exemplarylignin-zinc oxide nanohybrids were mixed with 25 mL of distilled waterand dispersed using ultrasonication. The sonicated lignin or thelignin-zinc oxide nanohybrid solution was mixed with an oil to form afirst mixture at a ratio of 100 mL sonicated lignin or lignin-zinc oxidenanohybrid solution to 5 mL oil. Various oils were used in exemplarysynthesis methods of the present disclosure, the oils being chosen fromcoconut oil, castor oil, sunflower oil, sesame oil, olive oil, and cornoil. In particular, in the following Examples 2-3 showingcharacterization studies and UV protection, coconut oil was used. Itshould be understood that an oil other than those listed may be selectedto best suit the ultimate desired application. A surfactant was added tothe first mixture to form a second mixture. The surfactant was chosenfrom either Tween 20 or Tween 80 (Tweens are polyethylene orpolyoxyethylene sorbitol ester derivatives, either laureates [Tween 20]or oleates [Tween 80]) in the exemplary synthesis methods performed, butit should be understood that a surfactant other than those mentioned maybe selected to stabilize the oil-water emulsion. Subsequently, thesecond mixture was sonicated for 30 minutes at 750 W using a probesonicator, producing a lignin emulsion used as a control or thelignin-zinc oxide nanohybrid emulsion (or nanoemulsion), which appearedas a light brown colloidal formulation for which UV shielding behaviorwas assessed, as in the following additional examples.

Example 2 Characterization of Lignin Nanohybrids and Lignin NanohybridNanoemulsion

The morphology and chemical composition of the lignin emulsion and thelignin-zinc oxide nanohybrid emulsion were analyzed using Field EmissionScanning Electron Microscopy (FE-SEM) and Energy-Dispersive X-raySpectroscopy (EDX) respectively. The formulation droplet size andstability was assessed using Dynamic Light Scattering (DLS) analysis.

UV spectra of the exemplary lignin-zinc oxide nanohybrids and theexemplary lignin-zinc oxide nanohybrid emulsion are shown in FIGS.1A-1B. The lignin nanohybrids showed a peak at 275 nm, presumably due tointrinsic isolated lignin architecture (FIG. 1A). Also, the exemplarylignin-zinc oxide nanohybrid emulsion showed a broader peak between290-300 nm.

The crystalline nature of the exemplary extracted lignin and exemplarylignin nanohybrids was assessed by XRD, as shown in FIG. 2. Thelignin-zinc oxide nanohybrid data (dark grey) shows several sharp peaksat 20 values of 31.6°, 34.4°, 36.2°, 47.3°, 56.4°, 62.7° and 68.9°,which correspond to ZnO lattice planes (100), (002), (101), (102),(110), (103), (112) and (201), respectively. A broad peak between10°-20° appearing in the lignin alone (light grey) and lignin-zinc oxidenanohybrids is attributed to lignin polymer.

The elemental composition of the lignin-zinc oxide nanohybrids isfurther illustrated and confirmed in FIGS. 3A-3B. The results of EDXanalysis of the exemplary lignin-zinc oxide nanohybrids clearly indicatethe presence of C, O and Zn, consistent with the makeup of lignin andZnO. No other elements indicative of impurities were observed. FIGS.3A-3B show the EDX mapping of the elements dispersion. These resultsshow that the ZnO appears distributed throughout the lignin evenly.Table 1 shows the relative abundance of C, O and Zn according to the EDXanalysis, showing that the lignin nanohybrids contain 5.95% Zn by weightpercentage. Thus, the lignin-zinc oxide nanohybrids are made of arelative large quantity of lignin and a small quantity of ZnO.

TABLE 1 Chemical Composition of Lignin Nanohybrids Element Weight %Atomic % C K 48.51 57.89 O K 45.54 40.80 Zn L 5.95 1.30

The structure and morphology of the exemplary lignin nanohybrids wereinvestigated using field emission scanning electron microscopy (FE-SEM).FIG. 6 depicts FE-SEM images of the exemplary lignin nanohybrids. Theseresults indicate 50-100 nm size ZnO nanostructures are attached to alignin matrix. Under ultrasonication, lignin presumably reduces andstabilizes the ZnO nanoparticles due to their free aliphatic hydroxyland phenolic hydroxyl groups. The particle size distribution determinedby DSL is shown in FIG. 7. The result shows lignin nanohybrids averageparticle size is 172 nm. The physicochemical analyses cumulativelyconfirm lignin/ZnO nanohybrid formation.

Example 3 UV Blocking Activity of Lignin Nanoemulsion and LigninNanohybrid Nanoemulsion

Furthermore, the exemplary lignin nanohybrids were used as a platformfor a UV-blocking formulation by incorporation into an oil nanoemulsion,as discussed previously. In the following examples, the oil used wascoconut oil. Exemplary lignin nanohybrid nanoemulsion, exemplary ligninnanoemulsion, coconut oil and isolated lignin were each prepared andtheir respective UV blocking activities were tested as follows (FIG. 8).

Lignin-zinc oxide nanohybrid emulsion, lignin emulsion, oil alone orisolated lignin alone was applied to 3M Transpore Tape adhered to aclean quartz plate; for each measurement, one of lignin nanohybridnanoemulsion, lignin nanoemulsion, coconut oil and isolated lignin werespread over the entire surface of the 3M Transpore Tape adhered to theclean quartz plate slowly with a thimble-coated finger. The samplecoated tape was then dried in a dark room for 30 min. UV transmittancewas measured using a UV spectrophotometer equipped with a solid sampleholder (Agilent Technologies, USA).

The lignin-zinc oxide nanohybrids absorbed around 80% of UV radiation,whereas the lignin-zinc oxide nanohybrid emulsion absorbed more than 95%of UV radiation (FIG. 7). Moreover, the lignin-zinc oxide nanohybridemulsion absorbed in UVA and UVB wavelengths. In contrast, coconut oiland lignin alone absorbed around 40% and 65% of UV radiation,respectively. Clearly the lignin-zinc oxide nanohybrid emulsioneffectively blocks a broad spectrum of UV radiation.

The lignin-zinc oxide nanohybrid emulsion is derived from naturallyavailable materials and is, thus, non-toxic, green, and biocompatible,in addition to effectively blocking a broad spectrum of UV radiation.The results indicate that the lignin-zinc oxide nanohybrid emulsion maybe suitable for sunscreens or other UV protection.

It is to be understood that the lignin-zinc oxide nanohybrid emulsionfor UV protection is not limited to the specific embodiments describedabove, but encompasses any and all embodiments within the scope of thegeneric language of the following claims enabled by the embodimentsdescribed herein, or otherwise shown in the drawings or described abovein terms sufficient to enable one of ordinary skill in the art to makeand use the claimed subject matter.

We claim:
 1. A lignin-zinc oxide nanohybrid emulsion for UV protection,comprising: an aqueous solution of a lignin-zinc oxide nanohybrid,wherein the lignin-zinc oxide nanohybrid comprises lignin extracted fromPhoenix dactylifera and has an average particle size of 172 nm, furtherwherein the aqueous solution has been sonicated for forty-five minutes;a biocompatible oil mixed with the aqueous solution of a lignin-zincoxide nanohybrid to form an emulsion, wherein the emulsion has beensonicated for thirty minutes; and a surfactant added to the emulsion tostabilize the emulsion.
 2. The lignin-zinc oxide nanohybrid emulsionaccording to claim 1, wherein the Phoenix dactylifera is obtained fromdate palms grown in Saudi Arabia.
 3. The lignin-zinc oxide nanohybridemulsion according to claim 1, wherein the biocompatible oil comprisesan oil selected from the group consisting of coconut oil, castor oil,sunflower oil, sesame oil, olive oil, and corn oil.
 4. The lignin-zincoxide nanohybrid emulsion according to claim 1, wherein thebiocompatible oil comprises coconut oil.
 5. The lignin-zinc oxidenanohybrid emulsion according to claim 1, wherein the surfactant isselected from the group consisting of polyethylene glycol soritanmonolaurate and polyethylene glycol soritan monooleate.